Is there an out-of-the-box thread pool with multiple queues (that ensure serial processing of each queue)?

Question

Among all my tasks, I have some that must be processed serially (they can never run concurrently and they must be processed in order).

I achieved that creating a sep

Answer 1

I recently answered a question about a "serial task queue" with a basic implementation as demonstration here. I imagine you have been using a similar solution. It is relatively easy to adapt the implementation to use a map of task lists and still share one (fixed size) executor.
The Striped Executor Service you mention is the better solution, but I show the adapted implementation here to demonstrate decoupling the task queue(s) from the executor. The implementation uses a callback and therefor has no need to do polling or signalling. Since a "critical (stop the world) section" is used, the map with task queues can clean itself: no tasks queued means empty map. Downside of the "critical section" is that throughput is limited: only so many tasks can be added and removed per second.

import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.ReentrantLock;

// Copied and updated from https://stackoverflow.com/a/32916943/3080094
public class SerialTaskQueues {

    public static void main(String[] args) {

        // test the serial task execution using different groups
        ExecutorService executor = Executors.newFixedThreadPool(2);
        SerialTaskQueues tq = new SerialTaskQueues(executor);
        try {
            // test running the tasks one by one
            tq.add(new SleepSome("1", 30L));
            Thread.sleep(5L);
            tq.add(new SleepSome("2", 20L));
            tq.add(new SleepSome("1", 10L));

            Thread.sleep(100L);
            // all queues should be empty
            System.out.println("Queue size 1: " + tq.size("1")); // should be empty
            System.out.println("Queue size 2: " + tq.size("2")); // should be empty
            tq.add(new SleepSome("1", 10L));
            tq.add(new SleepSome("2", 20L));
            // with executor pool size set to 2, task 3 will have to wait for task 1 to complete
            tq.add(new SleepSome("3", 30L));
            tq.add(new SleepSome("1", 20L));
            tq.add(new SleepSome("2", 10L));

            Thread.sleep(100L);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            executor.shutdownNow();
        }
    }

    // all lookups and modifications to the list must be synchronized on the list.
    private final Map taskGroups = new HashMap<>();
    // make lock fair so that adding and removing tasks is balanced.
    private final ReentrantLock lock = new ReentrantLock(true);
    private final ExecutorService executor;

    public SerialTaskQueues(ExecutorService executor) {
        this.executor = executor;
    }

    public boolean add(String groupId, Runnable task) {

        lock.lock();
        try {
            GroupTasks gt = taskGroups.get(groupId);
            if (gt == null) {
                gt = new GroupTasks(groupId);
                taskGroups.put(groupId, gt);
            }
            gt.tasks.add(task); 
        } finally {
            lock.unlock();
        }
        runNextTask(groupId);
        return true;
    }

    /* Utility method for testing. */
    public void add(SleepSome sleepTask) {
        add(sleepTask.groupId, sleepTask);
    }

    private void runNextTask(String groupId) {

        // critical section that ensures one task is executed.
        lock.lock();
        try {
            GroupTasks gt = taskGroups.get(groupId);
            if (gt.tasks.isEmpty()) {
                // only cleanup when last task has executed, prevent memory leak
                if (!gt.taskRunning.get()) {
                    taskGroups.remove(groupId);
                }
            } else if (!executor.isShutdown() && gt.taskRunning.compareAndSet(false, true)) {
                executor.execute(wrapTask(groupId, gt.taskRunning, gt.tasks.remove(0)));
            }
        } finally {
            lock.unlock();
        }
    }

    private CallbackTask wrapTask(final String groupId, final AtomicBoolean taskRunning, Runnable task) {

        return new CallbackTask(task, new Runnable() {
            @Override 
            public void run() {
                if (!taskRunning.compareAndSet(true, false)) {
                    System.out.println("ERROR: programming error, the callback should always run in execute state.");
                }
                runNextTask(groupId);
            }
        });
    }

    /** Amount of (active) task groups. */
    public int size() {

        int size = 0;
        lock.lock();
        try {
            size = taskGroups.size();
        } finally {
            lock.unlock();
        }
        return size;
    }

    public int size(String groupId) {

        int size = 0;
        lock.lock();
        try {
            GroupTasks gt = taskGroups.get(groupId);
            size = (gt == null ? 0 : gt.tasks.size());
        } finally {
            lock.unlock();
        }
        return size;
    }

    public Runnable get(String groupId, int index) {

        Runnable r = null;
        lock.lock();
        try {
            GroupTasks gt = taskGroups.get(groupId);
            r =  (gt == null ? null : gt.tasks.get(index));
        } finally {
            lock.unlock();
        }
        return r;
    }

    public Runnable remove(String groupId, int index) {

        Runnable r = null;
        lock.lock();
        try {
            GroupTasks gt = taskGroups.get(groupId);
            r = gt.tasks.remove(index);
            // similar to runNextTask - cleanup if there are no tasks (running) for the group 
            if (gt.tasks.isEmpty() && !gt.taskRunning.get()) {
                taskGroups.remove(groupId);
            }
        } finally {
            lock.unlock();
        }
        return r;
    }

    /* Helper class for the task-group map. */
    class GroupTasks {

        final List tasks = new LinkedList();
        // atomic boolean used to ensure only 1 task is executed at any given time
        final AtomicBoolean taskRunning = new AtomicBoolean(false);
        final String groupId;

        GroupTasks(String groupId) {
            this.groupId = groupId;
        }
    }

    // general callback-task, see https://stackoverflow.com/a/826283/3080094
    static class CallbackTask implements Runnable {

        private final Runnable task, callback;

        public CallbackTask(Runnable task, Runnable callback) {
            this.task = task;
            this.callback = callback;
        }

        @Override 
        public void run() {

            try {
                task.run();
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    callback.run();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }

    // task that just sleeps for a while
    static class SleepSome implements Runnable {

        static long startTime = System.currentTimeMillis();

        private final String groupId;
        private final long sleepTimeMs;
        public SleepSome(String groupId, long sleepTimeMs) {
            this.groupId = groupId;
            this.sleepTimeMs = sleepTimeMs;
        }
        @Override public void run() {
            try { 
                System.out.println(tdelta(groupId) + "Sleeping for " + sleepTimeMs + " ms.");
                Thread.sleep(sleepTimeMs);
                System.out.println(tdelta(groupId) + "Slept for " + sleepTimeMs + " ms.");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        private String tdelta(String groupId) { return String.format("% 4d [%s] ", (System.currentTimeMillis() - startTime), groupId); }
    }
}